Diving bradycardia is a reduction in the heart rate mediated by the parasympathetic system during diving. Although diving bradycardia is pronounced in aquatic mammals and birds, the existence of this response in aquatic reptiles, including sea turtles, remains under debate. Using the parasympathetic blocker atropine, we evaluated the involvement of the parasympathetic nervous system in heart rate reduction of loggerhead sea turtles (Caretta caretta) during voluntary diving in tanks. The heart rate of the control group dropped by 40–60% from the pre-dive value at the onset of diving; however, administration of atropine significantly inhibited heart rate reduction (P<0.001). Our results indicate that, similar to mammals and birds, the heart rate reduction in sea turtles while diving is primarily mediated by the parasympathetic nervous system. In conclusion, we suggest that diving bradycardia exists not only in aquatic mammals and birds but also in aquatic reptiles.